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Physical Review Letters

Electron correlations, magnetism, and structure of small clusters

G. M. Pastor, R. Hirsch, Bernhard Mühlschlegel

Published Jun 13, 1994• 72 citations
Publisher

Abstract

Many-body properties of N-atom clusters having N\ensuremath{\le}8 are calculated exactly in the framework of the Hubbard model for all possible nonequivalent cluster structures. The most stable structure and the corresponding total spin S are obtained rigorously as a function of the Coulomb-interaction strength U/t and the number of electrons. Results for the stability of cluster ferromagnetism against electronic excitations and structural changes are obtained for the first time. The resulting interplay between electron correlations, magnetism, and cluster structure is determined.

DOI
10.1103/physrevlett.72.3879

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Many-body properties of N-atom clusters having N\ensuremath{\le}8 are calculated exactly in the framework of the Hubbard model for all possible nonequivalent cluster structures. The most stable structure and the corresponding total spin S are obtained rigorously as a function of the Coulomb-interaction strength U/t and the number of electrons. Results for the stability of cluster ferromagnetism against electronic excitations and structural changes are obtained for the first time. The resulting interplay between electron correlations, magnetism, and cluster structure is determined.
The electronic, magnetic, and structural properties of small clusters are studied in the framework of the single-band Hubbard Hamiltonian. Results for various ground-state and excited-state many-body properties are presented, which were calculated exactly by means of Lanczos's numerical diagonalization method. A full geometry optimization is performed for N\ensuremath{\le}8 atoms by considering all possible nonequivalent cluster structures with fixed nearest-neighbor bond lengths. The most stable structure and the corresponding total spin S are obtained rigorously as a function of the Coulomb interaction strength U/t and number of electrons \ensuremath{\nu}. The resulting interplay between electron correlations, magnetism, and cluster structure is analyzed and the main trends as a function of N, U/t, and \ensuremath{\nu} are derived. The stability of cluster ferromagnetism is studied from two complementary points of view. First, for N\ensuremath{\le}8 and \ensuremath{\nu}=N+1, we determine exactly the stability of the ferromagnetic ground state with respect to electronic excitations and structural changes. It is shown that in small clusters the structural changes can be as important to the temperature dependence of the magnetization as the purely electronic excitations. Second, we determine the stability of the saturated ferromagnetic state with respect to single spin flips as a function of the band filling \ensuremath{\nu}/N. In this case a few selected larger clusters (7\ensuremath{\le}N\ensuremath{\le}43) in the strongly correlated limit (U/t\ensuremath{\rightarrow}+\ensuremath{\infty}) are considered. It is shown that the \ensuremath{\nu}/N dependence of the spin-flip energy \ensuremath{\Delta}${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{sf}}$ shows interesting electronic-shell-like oscillations, which reflect the characteristics of the single-particle energy-level structure and its dependence on the symmetry and size of the cluster. Finally, we conclude by discussing some of the limitations of the model together with relevant extensions. \textcopyright{} 1996 The American Physical Society.
The ground states of bcc, fcc, hcp, and icosahedral clusters having N=12−−14N=12--14N=12−−14 atoms are calculated exactly in the framework of the Hubbard Hamiltonian by using Lanczos's numerical diagonalization method. The total spin S shows a diversity of remarkable, sometimes surprising behaviors as a function of Coulomb interaction strength U/tU/tU/t and band filling $\ensuremath{\nu}/N.$ The magnetic properties are analyzed, in particular, by relating them to the corresponding single-particle spectra. Changes of symmetry close to real-space shell closings at N=13N=13N=13 are discussed. The relative stability of the different cluster geometries is determined and the corresponding structural and magnetic diagrams are derived. Electron correlation effects close to half-band filling result in changes of structure that are often accompanied by important changes in the magnetic behavior. Ferromagnetism is found to be particularly stable for $\ensuremath{\nu}/N\ensuremath{\simeq}1.2--1.4.$ The main trends as a function of $N, \ensuremath{\nu}$ and U/tU/tU/t are compared with previous exact diagonalization studies on smaller clusters.
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The ground-state and thermodynamic properties of a simple Hubbard Hamiltonian are studied for an eight-site cubic cluster as functions of the number of electrons and the strength of the electron interaction. The ground-state energy and spin are determined, and the excitation spectrum is computed. Thermodynamic properties including the specific heat, the magnetic susceptibility, and spin correlation functions are calculated. Computations are performed both by explicit diagonalization of the Hamiltonian and by Monte Carlo simulation.
By using a numerical exact diagonalization method, the properties of atomic clusters with atom number N=5 and N=6 are investigated in the framework of the extended Hubbard model. The optimized structure and the corresponding total spin S are obtained both as a function of Hubbard onsite interaction U, nearest-neighbor Coulomb repulsion V and filling number of electrons. Results show that with V increasing, the optimized structure of cluster turns into a chain or star-like structure with fewer bonds. The thermodynamic properties are also analyzed based on the distribution of energy levels of cluster with an optimized structure.
A review of the physical properties of small systems described by various model Hamiltonians is given. Their physical properties are found either exactly or by using Monte-Carlo methods, and are compared with properties of bulk systems. It is shown that (i) clusters differ in an essential way from the bulk, having electronic states strongly dependent on the geometry and cluster size, but, at the same time, (ii) important insights into the nature of electron correlation and low-energy excitations in solids may be gained by cluster studies. In particular, the cluster calculations allow one to identify the tendencies to various kinds of symmetry-broken states in the (extended) Hubbard model and give a simple qualitative understanding of the low temperature properties of the Kondo and the heavy-fermion systems. Furthermore, cluster calculations allow one to study such problems as the development of magnetic moment at chemisorbed atoms for strong correlations and the suppression of frustration by quantum fluctuations. Recently, cluster calculations gave important information about the possible role of charge transfer processes and magnetic interactions in the microscopic mechanism of pairing in high temperature superconducting oxides.